In a cellular communications network, user equipment (UE) (such as mobile telephones, mobile devices, mobile terminals, etc.) can communicate with other user equipment and/or remote servers via base stations. LTE systems include an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and an Evolved Packet Core (EPC) network (or simply ‘core network’). The E-UTRAN includes a number of base stations (‘eNBs’) for providing both user-plane (e.g. Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), Medium Access Control (MAC) and PHYsical (PHY) layers) and control-plane (e.g. Radio Resource Control (RRC)) protocol terminations towards the UE.
In order to provide seamless connectivity for the mobile devices, the base stations are configured with a list of their neighbour base stations so that the mobile devices can be handed over to one of the cells operated by other base stations when necessary (e.g. due to mobility of the mobile devices and/or changes in signal conditions and/or load balancing, etc). Therefore, each base station is required to store information relating to its neighbours including, inter alia, identifiers of the cells operated by each (known) neighbour base station, a unique identifier (e.g. eNB Id) associated with each neighbour base station, and a respective transport network layer (TNL) address associated with each neighbour base station. The TNL address facilitates communication between base stations via a so-called X2 interface, which is provided between each neighbour base station pair. The X2 interface uses the Stream Control Transmission Protocol (SCTP) to transmit data between the base stations.
Each base station can obtain the TNL address associated with another base station by following a so-called TNL Address Discovery procedure specified in section 22.3.6 of 3GPP TS 36.300, the contents of which are incorporated herein by reference. In summary, whenever a particular base station discovers a ‘candidate’ neighbour base station, it can request the so-called Mobility Management Entity (MME) in the core network to transfer configuration information between the two base stations via an S1 interface (which is provided between each base station and the core network). This procedure needs to be followed whenever there is a change in the configuration of one of the base stations and/or whenever a base station or a cell is added to (or removed from) the network to prevent handover problems for the mobile devices (e.g. incorrect selection of a handover target cell, which might result in loss of connection) in the vicinity of such cells. Since conventional (macro) base stations operate in an always-on mode and their configuration does not change often, this procedure does not cause unnecessary load on the core network elements and the S1 interface between the eNBs and the MME.
The 3GPP standards body has adopted an official architecture and defined standards for home base stations (‘HNB’). Where a home base station is operating in accordance with the LTE standards, the home base station is sometimes referred to as a HeNB. A similar architecture is also applied in the WiMAX network. In this case, the home base station is commonly referred to as a femto cell. For simplicity, the present application will use the term HeNB to refer to any such home base station and will use the term eNB generically to refer to other base stations (such as the base station for the macro cell in which a HeNB operates). The HeNB can provide radio coverage (for example, 3G/4G/WiMAX) via one of more cells within a home, small and medium enterprise environment, and/or in public places (such as shopping malls and the like). The HeNB connects to the core network via a suitable public network (for example via an ADSL link to the Internet) or operator network and in the case of the 3GPP standards, via a so called small cell gateway (e.g. including the functionality of a so called HeNB-GW) which typically aggregates traffic from several HeNBs.
Network operators are facing a number of challenges due to HeNB deployment. For example, the HeNBs are typically under the operational control of a customer rather than the network operator. Unlike eNBs, the home base stations may power on and off frequently (e.g. gracefully for energy saving reasons and/or abruptly for any other reasons) causing frequent configuration changes in the neighbouring (home) base stations (i.e. to add/remove the cell(s) operated by these home base stations and/or to update the corresponding X2 connections). In a worst case scenario, after a power ON/Off cycle, the TNL Address of a HeNB may change (because the address assignment might be the responsibility of another provider, e.g. an internet service provider, ISP). Hence, e.g. in the morning/evening when it becomes active in a typical home environment, every HeNB triggers a TNL Address Discovery process on discovering each of its neighbours, which are most likely the same neighbours as before. Although each HeNB discovers a small number of neighbours only, on a national level it can be in the order of millions, depending on the number of households operating their own HeNB.
Another challenge resulting from the high number of HeNBs is that each base station (i.e. eNB/HeNB) needs to maintain a large number of X2 connections (i.e. one with each of its neighbour eNBs/HeNBs). In order to reduce the number of X2 connections to be maintained in an eNB, a so-called X2-Gateway (X2-GW) entity (which may form part of or may be separate from the small cell gateway) can be provided between the eNBs and a predetermined group of HeNBs. In particular, the X2-GW makes it possible for an eNB to establish a single X2 connection with the X2-GW for each of a plurality of HeNBs that are also connected to that X2-GW. In this case, the number of HeNBs currently deployed (or being turned on) in the network can change without requiring re-configuration of the X2 connection between the eNB and the X2-GW—thus reducing the amount of signalling with the MME. Whenever a HeNB is turned on and/or reconfigured, it contacts its associated X2-GW which can provide access to any eNB also connected to this X2-GW without requiring contacting the MME (as would be the case for establishing a direct X2 connection to that eNB). There are two types of X2-GWs, the first one is a ‘full-poxy’ gateway (one that terminates non-UE traffic and stores X2 associations between (H)eNB addresses) and the other one is a ‘routing-proxy’ gateway (one that only routes signalling messages between two endpoints, based on addresses indicated in the messages themselves, but does not maintain any association between them).
Signalling traffic to the CN needs to be minimised in order to ensure enough bandwidth and processing time availability for other important traffic. However, even when an X2-GW is used, the TNL Address Discovery cannot be avoided completely for at least the following reasons:                Before X2 Setup, there needs to be an SCTP Association between two peers, i.e. a source (H)eNB and a target (H)eNB.        For SCTP Association, a source (and/or the X2-GW acting as a proxy) has to know the TNL Address of the target.        A source (H)eNB currently employs the TNL Address discovery procedure to get the TNL Address of the target (H)eNB.        This requires a source to send an ‘eNB Configuration Transfer’ message on an S1-MME interface and wait for a reply.        Traffic to an MME means signalling traffic to the core network, which can cause saturation if uncontrolled given the sheer number of HeNBs that may be provided in the network and their tendency to employ a power on/off cycle.        
Furthermore, with the introduction of the X2-GW, there is a need to discover two TNL addresses instead of one, i.e. the address of the peer (H)eNB and also of the X2-GW.
It can be seen therefore, that in current systems there can be a large signalling load towards core network entities that is associated with frequent configuration changes of HeNBs and with TNL address discovery procedures.
Although for efficiency of understanding for those of skill in the art, the invention will be described in detail in the context of an LTE system, the principles of the invention can be applied to other systems (such as WiMAX) in which (home) base stations communicate via a signalling gateway with the corresponding elements of the system changed as required.